1. Field of Invention
The present disclosure relates in general to monitoring fatigue loading in a component of a wellhead system by sensing a magnetized portion of the component. The disclosure further relates to magnetizing the component in strategic locations and disposing sensors proximate the magnetized locations.
2. Description of Prior Art
Wellheads used in the production of hydrocarbons extracted from subterranean formations typically comprise a wellhead assembly attached at the upper end of a wellbore formed into a hydrocarbon producing formation. Wellhead assemblies usually provide support hangers for suspending strings of production tubing and casing into the wellbore. A string of casing usually lines the wellbore, thereby isolating the wellbore from the surrounding formation. The tubing typically lies concentric within the casing and provides a conduit therein for producing the hydrocarbons entrained within the formation. A production tree is usually provided atop a wellhead housing, and is commonly used to control and distribute the fluids produced from the wellbore and selectively provide fluid communication or access to the tubing, casing, and/or annuluses between strings of concentric tubing and casing.
Wellhead housings, especially those subsea, typically include an outer low pressure housing welded onto a conductor pipe, where the conductor pipe is installed to a first depth in the well, usually by driving or jetting the conductor pipe. A drill bit inserts through the installed conductor pipe for drilling the well deeper to a second depth so that a high pressure housing can land within the low pressure housing. The high pressure housing usually has a length of pipe welded onto its lower end that extends into the wellbore past a lower end of the conductor pipe. The well is then drilled to its ultimate depth and completed, where completion includes landing casing strings in the high pressure housing that lines the wellbore, cementing between the casing string and wellbore wall, and landing production tubing within the production casing.
Once in operation, forces externally applied to the wellhead assembly such as from drilling, completion, workover operations, waves, and sea currents, can generate bending moments on the high and low pressure housings. As the widths of the low and high pressure housings reduce proximate attachment to the conductor pipes, stresses can concentrate along this change of thickness. Over time, repeated bending moments and other applied forces can fatigue load components of the wellhead assembly. Thus the safety of using a wellhead after ten years of operation is sometimes questioned; which can lead to the expensive option of replacing the aged wellhead. Moreover, the inability to directly measure wellhead fatigue sometimes requires a higher class welding connection, which can be unnecessarily expensive. Monitoring fatigue in a wellhead assembly remains a challenge for the industry. Strain gages have been used for measuring strain in a wellhead assembly, but they often become detached when subjected to the harsh environment within a wellhead assembly. Excessive wires/cables were hard to handle for sensor communication under the subsea environment. Finite element models have been used for fatigue analysis, but most require a transfer function to extrapolate the measured load of riser which is connected to the wellhead. The lack of the real fatigue data from the field had contributed to the uncertainty of the finite element analysis result.